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<meta name="description" content="Dubbo的很多拓展都是由SPI加载的，包括Protocal、Cluster、LoadBalance等等。而为了保证在接口调用的时候，通过URL来加载指定的类（比如LoadBalance策略配置），Dubbo提供了自适应扩展的机制来保证。 首先会为接口生成一个具有代理功能的代码，并通过Javasist或者jdk来编译这段代码，最后通过反射创建代理类。 Dubbo生成的代理类会在方法里去调用，去加载">
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<meta property="og:description" content="Dubbo的很多拓展都是由SPI加载的，包括Protocal、Cluster、LoadBalance等等。而为了保证在接口调用的时候，通过URL来加载指定的类（比如LoadBalance策略配置），Dubbo提供了自适应扩展的机制来保证。 首先会为接口生成一个具有代理功能的代码，并通过Javasist或者jdk来编译这段代码，最后通过反射创建代理类。 Dubbo生成的代理类会在方法里去调用，去加载">
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<meta name="twitter:description" content="Dubbo的很多拓展都是由SPI加载的，包括Protocal、Cluster、LoadBalance等等。而为了保证在接口调用的时候，通过URL来加载指定的类（比如LoadBalance策略配置），Dubbo提供了自适应扩展的机制来保证。 首先会为接口生成一个具有代理功能的代码，并通过Javasist或者jdk来编译这段代码，最后通过反射创建代理类。 Dubbo生成的代理类会在方法里去调用，去加载">



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          <h1 class="post-title" itemprop="name headline">自适应扩展</h1>
        

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        <p>Dubbo的很多拓展都是由SPI加载的，包括Protocal、Cluster、LoadBalance等等。而为了保证在接口调用的时候，通过URL来加载指定的类（比如LoadBalance策略配置），Dubbo提供了自适应扩展的机制来保证。</p>
<p>首先会为接口生成一个具有代理功能的代码，并通过Javasist或者jdk来编译这段代码，最后通过反射创建代理类。</p>
<p>Dubbo生成的代理类会在方法里去调用，去加载实际的类，并调用。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt.class).getExtension(extName);</span><br></pre></td></tr></table></figure>

<p>@Adaptive 注解主要是加在方法上面的，加在方法上说明需要靠框架来生成代理功能的代码。</p>
<p>直接看一段最核心的代码，用来生成代理类代码的方法。</p>
<p>ExtensionLoader.createAdaptiveExtensionClassCode()</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span 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class="line">212</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">private</span> String <span class="title">createAdaptiveExtensionClassCode</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        StringBuilder codeBuilder = <span class="keyword">new</span> StringBuilder();</span><br><span class="line">        Method[] methods = type.getMethods();</span><br><span class="line">        <span class="keyword">boolean</span> hasAdaptiveAnnotation = <span class="keyword">false</span>;</span><br><span class="line">  			<span class="comment">// 判断方法有没有加注解，没加就直接抛错了</span></span><br><span class="line">        <span class="keyword">for</span> (Method m : methods) &#123;</span><br><span class="line">            <span class="keyword">if</span> (m.isAnnotationPresent(Adaptive.class)) &#123;</span><br><span class="line">                hasAdaptiveAnnotation = <span class="keyword">true</span>;</span><br><span class="line">                <span class="keyword">break</span>;</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="comment">// no need to generate adaptive class since there's no adaptive method found.</span></span><br><span class="line">        <span class="keyword">if</span> (!hasAdaptiveAnnotation)</span><br><span class="line">            <span class="keyword">throw</span> <span class="keyword">new</span> IllegalStateException(<span class="string">"No adaptive method on extension "</span> + type.getName() + <span class="string">", refuse to create the adaptive class!"</span>);</span><br><span class="line">				<span class="comment">// package，在接口类的目录下</span></span><br><span class="line">        codeBuilder.append(<span class="string">"package "</span>).append(type.getPackage().getName()).append(<span class="string">";"</span>);</span><br><span class="line">  		 <span class="comment">// import extensionLoader</span></span><br><span class="line">        codeBuilder.append(<span class="string">"\nimport "</span>).append(ExtensionLoader.class.getName()).append(<span class="string">";"</span>);</span><br><span class="line">  <span class="comment">// 自适应类在接口后面加 $Adaptive</span></span><br><span class="line">        codeBuilder.append(<span class="string">"\npublic class "</span>).append(type.getSimpleName()).append(<span class="string">"$Adaptive"</span>).append(<span class="string">" implements "</span>).append(type.getCanonicalName()).append(<span class="string">" &#123;"</span>);</span><br><span class="line"><span class="comment">// 遍历方法生成</span></span><br><span class="line">        <span class="keyword">for</span> (Method method : methods) &#123;</span><br><span class="line">            Class&lt;?&gt; rt = method.getReturnType();</span><br><span class="line">            Class&lt;?&gt;[] pts = method.getParameterTypes();</span><br><span class="line">            Class&lt;?&gt;[] ets = method.getExceptionTypes();</span><br><span class="line"><span class="comment">// 如果不包含adaptive注解，调用直接报错</span></span><br><span class="line">            Adaptive adaptiveAnnotation = method.getAnnotation(Adaptive.class);</span><br><span class="line">            StringBuilder code = <span class="keyword">new</span> StringBuilder(<span class="number">512</span>);</span><br><span class="line">            <span class="keyword">if</span> (adaptiveAnnotation == <span class="keyword">null</span>) &#123;</span><br><span class="line">                code.append(<span class="string">"throw new UnsupportedOperationException(\"method "</span>)</span><br><span class="line">                        .append(method.toString()).append(<span class="string">" of interface "</span>)</span><br><span class="line">                        .append(type.getName()).append(<span class="string">" is not adaptive method!\");"</span>);</span><br><span class="line">            &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">              <span class="comment">// 以下这段代码的主要流程就是</span></span><br><span class="line">              <span class="comment">// 1. 查找URL，如果参数里没有，就从参数的attribute里找。都没有就报错</span></span><br><span class="line">              <span class="comment">// 2. 从URL里获取参数名，如果没有配置，那么就根据类名获取，比如 SimpleExt-&gt; simple.ext</span></span><br><span class="line">              <span class="comment">// </span></span><br><span class="line">                <span class="keyword">int</span> urlTypeIndex = -<span class="number">1</span>;</span><br><span class="line">                <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; pts.length; ++i) &#123;</span><br><span class="line">                    <span class="keyword">if</span> (pts[i].equals(URL.class)) &#123;</span><br><span class="line">                        urlTypeIndex = i;</span><br><span class="line">                        <span class="keyword">break</span>;</span><br><span class="line">                    &#125;</span><br><span class="line">                &#125;</span><br><span class="line">                <span class="comment">// found parameter in URL type</span></span><br><span class="line">                <span class="keyword">if</span> (urlTypeIndex != -<span class="number">1</span>) &#123;</span><br><span class="line">                    <span class="comment">// Null Point check</span></span><br><span class="line">                    String s = String.format(<span class="string">"\nif (arg%d == null) throw new IllegalArgumentException(\"url == null\");"</span>,</span><br><span class="line">                            urlTypeIndex);</span><br><span class="line">                    code.append(s);</span><br><span class="line"></span><br><span class="line">                    s = String.format(<span class="string">"\n%s url = arg%d;"</span>, URL.class.getName(), urlTypeIndex);</span><br><span class="line">                    code.append(s);</span><br><span class="line">                &#125;</span><br><span class="line">                <span class="comment">// did not find parameter in URL type</span></span><br><span class="line">                <span class="keyword">else</span> &#123;</span><br><span class="line">                    String attribMethod = <span class="keyword">null</span>;</span><br><span class="line">    </span><br><span class="line">                    <span class="comment">// find URL getter method</span></span><br><span class="line">                    LBL_PTS:</span><br><span class="line">                    <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; pts.length; ++i) &#123;</span><br><span class="line">                        Method[] ms = pts[i].getMethods();</span><br><span class="line">                        <span class="keyword">for</span> (Method m : ms) &#123;</span><br><span class="line">                            String name = m.getName();</span><br><span class="line">                            <span class="keyword">if</span> ((name.startsWith(<span class="string">"get"</span>) || name.length() &gt; <span class="number">3</span>)</span><br><span class="line">                                    &amp;&amp; Modifier.isPublic(m.getModifiers())</span><br><span class="line">                                    &amp;&amp; !Modifier.isStatic(m.getModifiers())</span><br><span class="line">                                    &amp;&amp; m.getParameterTypes().length == <span class="number">0</span></span><br><span class="line">                                    &amp;&amp; m.getReturnType() == URL.class) &#123;</span><br><span class="line">                                urlTypeIndex = i;</span><br><span class="line">                                attribMethod = name;</span><br><span class="line">                                <span class="keyword">break</span> LBL_PTS;</span><br><span class="line">                            &#125;</span><br><span class="line">                        &#125;</span><br><span class="line">                    &#125;</span><br><span class="line">                    <span class="keyword">if</span> (attribMethod == <span class="keyword">null</span>) &#123;</span><br><span class="line">                        <span class="keyword">throw</span> <span class="keyword">new</span> IllegalStateException(<span class="string">"fail to create adaptive class for interface "</span> + type.getName()</span><br><span class="line">                                + <span class="string">": not found url parameter or url attribute in parameters of method "</span> + method.getName());</span><br><span class="line">                    &#125;</span><br><span class="line">    </span><br><span class="line">                    <span class="comment">// Null point check</span></span><br><span class="line">                    String s = String.format(<span class="string">"\nif (arg%d == null) throw new IllegalArgumentException(\"%s argument == null\");"</span>,</span><br><span class="line">                            urlTypeIndex, pts[urlTypeIndex].getName());</span><br><span class="line">                    code.append(s);</span><br><span class="line">                    s = String.format(<span class="string">"\nif (arg%d.%s() == null) throw new IllegalArgumentException(\"%s argument %s() == null\");"</span>,</span><br><span class="line">                            urlTypeIndex, attribMethod, pts[urlTypeIndex].getName(), attribMethod);</span><br><span class="line">                    code.append(s);</span><br><span class="line">    </span><br><span class="line">                    s = String.format(<span class="string">"%s url = arg%d.%s();"</span>, URL.class.getName(), urlTypeIndex, attribMethod);</span><br><span class="line">                    code.append(s);</span><br><span class="line">                &#125;</span><br><span class="line">    </span><br><span class="line">                String[] value = adaptiveAnnotation.value();</span><br><span class="line">                <span class="comment">// value is not set, use the value generated from class name as the key</span></span><br><span class="line">              <span class="comment">// 2. 从URL里获取参数名，如果没有配置，那么就根据类名获取，比如 SimpleExt-&gt; simple.ext</span></span><br><span class="line">              <span class="comment">// 如果指定了 @Adaptive(&#123;"key1", "key2"&#125;)，那么value就是['key1', 'key2']。后面会根据这两个key从URL里获取值</span></span><br><span class="line">                <span class="keyword">if</span> (value.length == <span class="number">0</span>) &#123;</span><br><span class="line">                    <span class="keyword">char</span>[] charArray = type.getSimpleName().toCharArray();</span><br><span class="line">                    StringBuilder sb = <span class="keyword">new</span> StringBuilder(<span class="number">128</span>);</span><br><span class="line">                    <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; charArray.length; i++) &#123;</span><br><span class="line">                        <span class="keyword">if</span> (Character.isUpperCase(charArray[i])) &#123;</span><br><span class="line">                            <span class="keyword">if</span> (i != <span class="number">0</span>) &#123;</span><br><span class="line">                                sb.append(<span class="string">"."</span>);</span><br><span class="line">                            &#125;</span><br><span class="line">                            sb.append(Character.toLowerCase(charArray[i]));</span><br><span class="line">                        &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">                            sb.append(charArray[i]);</span><br><span class="line">                        &#125;</span><br><span class="line">                    &#125;</span><br><span class="line">                    value = <span class="keyword">new</span> String[]&#123;sb.toString()&#125;;</span><br><span class="line">                &#125;</span><br><span class="line">    </span><br><span class="line">                <span class="keyword">boolean</span> hasInvocation = <span class="keyword">false</span>;</span><br><span class="line">                <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; pts.length; ++i) &#123;</span><br><span class="line">                    <span class="keyword">if</span> (pts[i].getName().equals(<span class="string">"com.alibaba.dubbo.rpc.Invocation"</span>)) &#123;</span><br><span class="line">                        <span class="comment">// Null Point check</span></span><br><span class="line">                        String s = String.format(<span class="string">"\nif (arg%d == null) throw new IllegalArgumentException(\"invocation == null\");"</span>, i);</span><br><span class="line">                        code.append(s);</span><br><span class="line">                        s = String.format(<span class="string">"\nString methodName = arg%d.getMethodName();"</span>, i);</span><br><span class="line">                        code.append(s);</span><br><span class="line">                        hasInvocation = <span class="keyword">true</span>;</span><br><span class="line">                        <span class="keyword">break</span>;</span><br><span class="line">                    &#125;</span><br><span class="line">                &#125;</span><br><span class="line">    						<span class="comment">// 生成 获取extension，并调用的代码</span></span><br><span class="line">                String defaultExtName = cachedDefaultName;</span><br><span class="line">                String getNameCode = <span class="keyword">null</span>;</span><br><span class="line">                <span class="keyword">for</span> (<span class="keyword">int</span> i = value.length - <span class="number">1</span>; i &gt;= <span class="number">0</span>; --i) &#123;</span><br><span class="line">                    <span class="keyword">if</span> (i == value.length - <span class="number">1</span>) &#123;</span><br><span class="line">                        <span class="keyword">if</span> (<span class="keyword">null</span> != defaultExtName) &#123;</span><br><span class="line">                            <span class="keyword">if</span> (!<span class="string">"protocol"</span>.equals(value[i]))</span><br><span class="line">                                <span class="keyword">if</span> (hasInvocation)</span><br><span class="line">                                    getNameCode = String.format(<span class="string">"url.getMethodParameter(methodName, \"%s\", \"%s\")"</span>, value[i], defaultExtName);</span><br><span class="line">                                <span class="keyword">else</span></span><br><span class="line">                                    getNameCode = String.format(<span class="string">"url.getParameter(\"%s\", \"%s\")"</span>, value[i], defaultExtName);</span><br><span class="line">                            <span class="keyword">else</span></span><br><span class="line">                                getNameCode = String.format(<span class="string">"( url.getProtocol() == null ? \"%s\" : url.getProtocol() )"</span>, defaultExtName);</span><br><span class="line">                        &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">                            <span class="keyword">if</span> (!<span class="string">"protocol"</span>.equals(value[i]))</span><br><span class="line">                                <span class="keyword">if</span> (hasInvocation)</span><br><span class="line">                                    getNameCode = String.format(<span class="string">"url.getMethodParameter(methodName, \"%s\", \"%s\")"</span>, value[i], defaultExtName);</span><br><span class="line">                                <span class="keyword">else</span></span><br><span class="line">                                    getNameCode = String.format(<span class="string">"url.getParameter(\"%s\")"</span>, value[i]);</span><br><span class="line">                            <span class="keyword">else</span></span><br><span class="line">                                getNameCode = <span class="string">"url.getProtocol()"</span>;</span><br><span class="line">                        &#125;</span><br><span class="line">                    &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">                        <span class="keyword">if</span> (!<span class="string">"protocol"</span>.equals(value[i]))</span><br><span class="line">                            <span class="keyword">if</span> (hasInvocation)</span><br><span class="line">                                getNameCode = String.format(<span class="string">"url.getMethodParameter(methodName, \"%s\", \"%s\")"</span>, value[i], defaultExtName);</span><br><span class="line">                            <span class="keyword">else</span></span><br><span class="line">                                getNameCode = String.format(<span class="string">"url.getParameter(\"%s\", %s)"</span>, value[i], getNameCode);</span><br><span class="line">                        <span class="keyword">else</span></span><br><span class="line">                            getNameCode = String.format(<span class="string">"url.getProtocol() == null ? (%s) : url.getProtocol()"</span>, getNameCode);</span><br><span class="line">                    &#125;</span><br><span class="line">                &#125;</span><br><span class="line">                code.append(<span class="string">"\nString extName = "</span>).append(getNameCode).append(<span class="string">";"</span>);</span><br><span class="line">                <span class="comment">// check extName == null?</span></span><br><span class="line">                String s = String.format(<span class="string">"\nif(extName == null) "</span> +</span><br><span class="line">                                <span class="string">"throw new IllegalStateException(\"Fail to get extension(%s) name from url(\" + url.toString() + \") use keys(%s)\");"</span>,</span><br><span class="line">                        type.getName(), Arrays.toString(value));</span><br><span class="line">                code.append(s);</span><br><span class="line">    </span><br><span class="line">                s = String.format(<span class="string">"\n%s extension = (%&lt;s)%s.getExtensionLoader(%s.class).getExtension(extName);"</span>,</span><br><span class="line">                        type.getName(), ExtensionLoader.class.getSimpleName(), type.getName());</span><br><span class="line">                code.append(s);</span><br><span class="line">    </span><br><span class="line">                <span class="comment">// return statement</span></span><br><span class="line">              <span class="comment">// 返回值</span></span><br><span class="line">                <span class="keyword">if</span> (!rt.equals(<span class="keyword">void</span>.class)) &#123;</span><br><span class="line">                    code.append(<span class="string">"\nreturn "</span>);</span><br><span class="line">                &#125;</span><br><span class="line">    </span><br><span class="line">                s = String.format(<span class="string">"extension.%s("</span>, method.getName());</span><br><span class="line">                code.append(s);</span><br><span class="line">                <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; pts.length; i++) &#123;</span><br><span class="line">                    <span class="keyword">if</span> (i != <span class="number">0</span>)</span><br><span class="line">                        code.append(<span class="string">", "</span>);</span><br><span class="line">                    code.append(<span class="string">"arg"</span>).append(i);</span><br><span class="line">                &#125;</span><br><span class="line">                code.append(<span class="string">");"</span>);</span><br><span class="line">            &#125;</span><br><span class="line">    <span class="comment">// 把代码拼接回code里</span></span><br><span class="line">            codeBuilder.append(<span class="string">"\npublic "</span>).append(rt.getCanonicalName()).append(<span class="string">" "</span>).append(method.getName()).append(<span class="string">"("</span>);</span><br><span class="line">            <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; pts.length; i++) &#123;</span><br><span class="line">                <span class="keyword">if</span> (i &gt; <span class="number">0</span>) &#123;</span><br><span class="line">                    codeBuilder.append(<span class="string">", "</span>);</span><br><span class="line">                &#125;</span><br><span class="line">                codeBuilder.append(pts[i].getCanonicalName());</span><br><span class="line">                codeBuilder.append(<span class="string">" "</span>);</span><br><span class="line">                codeBuilder.append(<span class="string">"arg"</span>).append(i);</span><br><span class="line">            &#125;</span><br><span class="line">            codeBuilder.append(<span class="string">")"</span>);</span><br><span class="line">            <span class="keyword">if</span> (ets.length &gt; <span class="number">0</span>) &#123;</span><br><span class="line">                codeBuilder.append(<span class="string">" throws "</span>);</span><br><span class="line">                <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; ets.length; i++) &#123;</span><br><span class="line">                    <span class="keyword">if</span> (i &gt; <span class="number">0</span>) &#123;</span><br><span class="line">                        codeBuilder.append(<span class="string">", "</span>);</span><br><span class="line">                    &#125;</span><br><span class="line">                    codeBuilder.append(ets[i].getCanonicalName());</span><br><span class="line">                &#125;</span><br><span class="line">            &#125;</span><br><span class="line">            codeBuilder.append(<span class="string">" &#123;"</span>);</span><br><span class="line">            codeBuilder.append(code.toString());</span><br><span class="line">            codeBuilder.append(<span class="string">"\n&#125;"</span>);</span><br><span class="line">        &#125;</span><br><span class="line">        codeBuilder.append(<span class="string">"\n&#125;"</span>);</span><br><span class="line">        <span class="keyword">if</span> (logger.isDebugEnabled()) &#123;</span><br><span class="line">            logger.debug(codeBuilder.toString());</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">return</span> codeBuilder.toString();</span><br><span class="line">    &#125;</span><br></pre></td></tr></table></figure>


      
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